This invention relates, in general, to radar systems and, more specifically, to radar processors having selectable doppler filters.
The echo processing techniques used with certain radar systems employ doppler filters which ideally should pass a narrow band of frequencies and totally reject all others. However, such an ideal filter is not obtainable. Actual doppler filters are normally implemented by processing a finite number of received signals or echoes in the digital processor of the radar receiver. These actual filters pass the desired narrow band of frequencies but also pass, to some extent, the signals whose frequencies are in the rejection band of the filter. The rejection band includes the signals associated with ground clutter echoes and with other doppler shifted echoes, such as those from moving rain. Both ground clutter and moving rain echoes are considered unwanted interference and tend to degrade the ability of the radar to detect an aircraft echo signal contained within the pass band of the filter.
In order to alleviate this condition, prior radar systems have used digital doppler filters which have been designed to cope with specific levels of the two types of interference. However, both types of interference vary widely, both spatially and temporally. If filters could be designed to handle the highest expected levels of both types of interference of clutter simultaneously, the problem would be greatly reduced. Unfortunately, typical dwell times on the target do not provide a sufficient number of pulses to provide such filters. Therefore, a compromise in filter response has been necessary according to the prior art. Adaptive digital doppler filters have been known in the prior art, but their use has been minimized because of their complexity and because of concern about their ability to maintain clutter suppression capability in the presence of strong moving targets and pulses from other radar systems and electronic counter measures (ECM).
At least one radar system uses a technique for switching between doppler filters wherein, at intervals of five minutes, the target data is interrupted for one scan to determine which filter provides the minimum output in each spatial zone. This selection process causes a loss of data for one scan and makes it difficult to track targets of marginal detection probability which may fade on the preceeding or succeeding scan. Such selection process also has an inability to react to changing rainfall and wind at a given spatial location over a five minute time interval and is susceptible to selecting the wrong filter in those zones where a moving target is detectable.
Several U.S. patents have been issued which teach various arrangements and methods for dealing with filter performance under varying conditions. U.S. Pat. No. 3,775,768, issued Nov. 27, 1973, describes a clutter filter which is adapted to the prevailing signal by changing the notch of the filter according to the measured amplitude and the expected bandwidth of ground clutter signals. U.S. Pat. No. 4,339,754, issued July 13, 1982, describes the selection between two moving target indicator (MTI) systems. Such selection is made by determining the degree of correlation of the echoes. U.S. Pat. No. 3,990,076, issued Nov. 2, 1976, describes a selection process in an angle-tracking radar whereby the echo amplitudes from range bins straddling the target range bin are sensed to determine if either magnitude warrants selecting the doppler filter which attenuates ground clutter. These and other U.S. patents which have been issued and teach changing the response of filters based upon the measurement of certain variables are useful in some situations; however, they all differ significantly from the teachings of the present invention.
To alleviate some of the problems associated with prior art doppler filter selecting arrangements, it is desirable, and it is an object of this invention, to provide an arrangement or device for selecting appropriate doppler filters without losing target data for one scan and for selecting the filter frequently enough to react favorably to clutter changes over a given spatial location.